net/http: in Client, consume small redirect bodies before making next request

In Go 1.2, closing a request body without reading to EOF
causes the underlying TCP connection to not be reused. This
client code following redirects was never updated when that
happened.

This was part of a previous CL but moved to its own CL at
Josh's request.  Now with test too.

LGTM=josharian
R=josharian
CC=golang-codereviews
https://golang.org/cl/70800043

src/pkg/net/http/client.go

@@ -14,6 +14,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"io/ioutil"
 	"log"
 	"net/url"
 	"strings"
@@ -337,6 +338,12 @@ func (c *Client) doFollowingRedirects(ireq *Request, shouldRedirect func(int) bo
 		}
 
 		if shouldRedirect(resp.StatusCode) {
+			// Read the body if small so underlying TCP connection will be re-used.
+			// No need to check for errors: if it fails, Transport won't reuse it anyway.
+			const maxBodySlurpSize = 2 << 10
+			if resp.ContentLength == -1 || resp.ContentLength <= maxBodySlurpSize {
+				io.CopyN(ioutil.Discard, resp.Body, maxBodySlurpSize)
+			}
 			resp.Body.Close()
 			if urlStr = resp.Header.Get("Location"); urlStr == "" {
 				err = errors.New(fmt.Sprintf("%d response missing Location header", resp.StatusCode))

src/pkg/net/http/client_test.go

@@ -879,3 +879,43 @@ func TestClientTimeout(t *testing.T) {
 		t.Errorf("timeout after %v waiting for timeout of %v", failTime, timeout)
 	}
 }
+
+func TestClientRedirectEatsBody(t *testing.T) {
+	defer afterTest(t)
+	saw := make(chan string, 2)
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		saw <- r.RemoteAddr
+		if r.URL.Path == "/" {
+			Redirect(w, r, "/foo", StatusFound) // which includes a body
+		}
+	}))
+	defer ts.Close()
+
+	res, err := Get(ts.URL)
+	if err != nil {
+		t.Fatal(err)
+	}
+	_, err = ioutil.ReadAll(res.Body)
+	if err != nil {
+		t.Fatal(err)
+	}
+	res.Body.Close()
+
+	var first string
+	select {
+	case first = <-saw:
+	default:
+		t.Fatal("server didn't see a request")
+	}
+
+	var second string
+	select {
+	case second = <-saw:
+	default:
+		t.Fatal("server didn't see a second request")
+	}
+
+	if first != second {
+		t.Fatal("server saw different client ports before & after the redirect")
+	}
+}